Single-cell techniques using chromosomally tagged fluorescent bacteria to study Listeria monocytogenes infection processes.

Listeria monocytogenes is a Gram-positive facultative intracellular pathogen which invades different cell types, including nonphagocytic cells, where it is able to replicate and survive. The different steps of the cellular infectious process have been well described and consist of bacterial entry, lysis of the endocytic vacuole, intracellular replication, and spreading to neighboring cells. To study the listerial infectious process, gentamicin survival assays, plaque formation, and direct microscopy observations are typically used; however, there are some caveats with each of these techniques. In this study we describe new single-cell techniques based on use of an array of integrative fluorescent plasmids (green, cyan, and yellow fluorescent proteins) to easily, rapidly, and quantitatively detect L. monocytogenes in vitro and in vivo. We describe construction of 13 integrative and multicopy plasmids which can be used for detecting intracellular bacteria, for measuring invasion, cell-to-cell spreading, and intracellular replication, for monitoring in vivo infections, and for generating transcriptional or translational reporters. Furthermore, we tested these plasmids in a variety of epifluorescence- and flow cytometry-based assays. We showed that we could (i) determine the expression of a particular promoter during the cell cycle, (ii) establish in one rapid experiment at which step in the cell cycle a particular mutant is defective, and (iii) easily measure the number of infected cells in vitro and in mouse organs. The plasmids that are described and the methods to detect them are new powerful tools to study host-Listeria interactions in a fast, robust, and high-throughput manner.